1. Field of the Invention
This invention relates to sintered ITO (indium-tin oxide) for use as a target in the formation of an ITO film by sputtering. The invention also relates to a target made of such sintered ITO.
2. Background Information
An ITO film is one of the most common light-transmissive and electrically conductive films and it finds extensive use as transparent electrodes in liquid-crystal displays, solar cells and other optoelectronic devices. While ITO films can be fabricated by various methods including spraying, vacuum evaporation, ion plating and sputtering, the last-mentioned method has drawn particular attention since it is capable of producing ITO films of good quality at high yield.
In proportion as displaying devices tend to have more and more increased areas and become more and more sophisticated, the needs for ITO films having decreased resistance and increased light transmittance is becoming greater. Thus, with respect to the sputtering in which a sintered ITO is used as a target, study has been made to solve problems such as how to secure the stability of film formation during sputtering, how to restrain the formation of particles, how to avoid the blackening of the target surface and how to suppress the formation of nodules in order to obtain films satisfying the above stated requirements.
It is known that by decreasing the porosity of sinters, i.e. by increasing the density thereof, the occurrence of blackening and the formation of nodules during the sputtering can be suppressed effectively to ensure the stable formation of sputterred films.
In order to produce an ITO film of low specific resistance using a sintered ITO target, it is desired that the dissolution of carrier Sn and the oxygen holes which also work as carriers should be kept within appropriate ranges that allow a homogeneous ITO film to be formed in a consistent manner. To this end, sputtering is usually performed with a mixture of argon and oxygen gases being used such as to control the conditions for forming a desired ITO film. In this case, the proportion of oxygen in the mixed gas affects the specific resistance of the resulting ITO film as shown in FIG. 1. If the proportion of oxygen is unduly low, the film forming atmosphere is slightly in a reducing state and substances of high electric resistance such as InO will form to increase the specific resistance of the ITO film being produced. In addition, metallization and the formation of whiskers occur in an oxygen-lean region to reduce the light transmittance of the film. On the other hand, if the proportion of oxygen is unduly high, oxygen holes working as carriers will decrease to eventually increase the specific resistance of the film. Therefore, maintaining the proportion of oxygen within an optimal range such that the specific resistance of the ITO film being formed is held consistently low is critical to successful sputtering. A problem to be considered here is that if an optimal range for the proportion of oxygen is narrow, it is difficult to form an ITO film of low specific resistance in a consistent manner.
Indium-tin oxide (ITO) sputtering targets are used in the formation of transparent, electrically conductive films. Such targets are customarily produced by sintering a compact of a tin oxide containing indium oxide powder which is either a mixture of indium oxide and tin oxide powders or derived from a coprecipitated powder. Examples of the starting materials for ITO production include an indium oxide powder, a tin oxide powder, a mixture of indium oxide and tin oxide powders, and a tin oxide containing indium oxide powder derived from a coprecipitated powder. To prepare an indium oxide powder, an aqueous solution of an indium salt as a starting material for synthesis is reacted with an aqueous solution of ammonia or an alkali hydroxide such as sodium hydroxide to make indium hydroxide, which is washed with water, dried and sintered. If a tin salt is incorporated in the aqueous indium salt solution, a tin-containing coprecipitated powder will result and from this powder, a tin oxide containing indium oxide powder can also be obtained.
The tin content of ITO is variable with the conditions of film formation and the desired film characteristics; typically, it ranges from 2 to 20 wt %, particularly from 4 to 15 wt %, in terms of SnO.sub.2.
Sintered ITO for use as a sputtering target has commonly been produced by a cold press sintering process which comprises the steps of pressing a starting powder into a compact at ordinary temperature, sintering the compact and machining it to a final form.
Commonly, however, indium oxide powder or tin oxide powder is poor in sinterability. They can not be sintered into high density sinter products when sintered in an ordinary open air atmosphere. As disclosed in Unexamined Published Japanese Patent Application No. Hei 5-209264, there is a proposed sintering method in which an oxygen partial pressure is controlled. This process could not produce a sinter of sufficiently high density, when performed under low oxygen partial pressure.
Other proposals for the production of high-density sinters include hot pressing, HIP (hot isostatic pressing) and other methods in which pressure is applied under hot conditions to allow for an increase in the density of the sinter being formed.
Still other proposals for the production of high-density sinters include a method as disclosed in the Examined Published Japanese Patent Application No. Hei 5-30905 in which sintering is performed in the pressurized oxygen atmosphere in order to suppress the dissociation of In.sub.2 O.sub.3 to thereby increase the density of sinters.
Hot pressing, HIP and the sintering in pressurized oxygen require more investment than normal sintering and hence involves increased manufacturing costs, thereby imposing constraints on the effort to produce larger targets. The present inventors have proposed a method as disclosed in the Unexamined Published Japanese Patent Application No. Hei 6-48816 in which high-density sinters are obtained by adjusting the powder characteristics of the starting materials instead of using particularly pressurized oxygen atmosphere. In this process, however, it is not easy to obtain the products having such optimal oxygen content as will be described hereinafter. A process in which low-density sinters are heat treated in an oxygen-free atmosphere such as in vacuo to thereby control the oxygen content of sinters has also been proposed.